This invention relates to an optical deflector, which is used for light-scanning in image recording apparatus such as laser printers, a method of assembling the deflector, an adjusting method of the deflector, and an apparatus of adjusting the deflector.
In image recording apparatus such as laser printers, the image recording is made according to the image information, which has been read, in the following manner: a laser beam is made to be incident on a polygonal mirror and to reflect to be projected onto a photoreceptor surface to scan it. FIG. 4 shows a perspective view of the writing optical system equipped with the optical deflector 100. The laser beam emitted from the semiconductor laser 1 becomes a parallel pencil of light by the collimator lens 1 of the beam-shape regulating optical unit, passing through the first cylindrical lens 3, and is reflected to be deflected by the polygonal mirror 101 of the optical deflector 100; then, it passes through the F.theta. lens 4 and the second cylindrical lens 5 to be projected onto the photoreceptor drum 6 via the reflecting mirror 9 for scanning the photoreceptor with a predetermined spot diameter in the primary scanning direction. The synchronizing per every scanning line is detected by causing the laser beam before starting to scan the line to enter the synchronization detector 8 via the mirror 7.
In the scanning optical system like this, a very small error of the tilt angle of the polygonal mirror results in the fluctuation of the scanning line and the distortion of the image, deteriorating the image quality. This becomes more remarkable with the higher rotational speed of the polygonal mirror for the higher recording density.
The polygonal mirror is directly fixed to the rotary shaft of the motor to be used at lower rotational speeds, however, in case of higher rotational speeds, the driving-rotating with an air bearing is practiced; that is, the polygonal mirror is fixed to the outer ring of the radial bearing and rotates floating without contacting the inner ring of the radial bearing. The applicant has disclosed the technology relating to the optical deflector having a hydrodynamic bearing part in the Japanese laid open patents H7-243437, H7-259849, H8-114219, H8-121471, and so forth; FIG. 5 is a cross-sectional view showing the structure of the optical deflector having the hydrodynamic bearing part composed of the upper thrust plate 124, lower thrust plate 123, and the inner ring 125. Referring to FIG. 5, the inner ring 125, the lower thrust plate 123, and the coil 126 for producing the static magnetic field, all arranged concentrically and united with the base plate 121, make up the hydrodynamic bearing part 120. Further, the ring-shaped magnet 106, the outer ring 104 made of aluminum, the outer ring 102 made of a ceramic material, the polygonal mirror 101, and the mirror holder 103, with the polygonal mirror nipped by the outer ring 104 and the mirror holder 103 like a sandwich, all arranged concentrically and assembled unitedly, make up the rotor 110. The rotor 110 is fitted to the outside of the aforesaid inner ring 125, and then the upper thrust plate 124 is fixed concentrically to the inner ring 125. In addition, small gaps having spacings of 3-10 .mu.m are formed between the opposite surfaces, that is, between the peripheral surface of the inner ring and the inner surface of the outer ring, between the bottom surface of the upper thrust plate 124 and the top surface of the outer ring 102, and between the top surface of the lower thrust plate 123 and the bottom surface of the outer ring 102; when rotating, the rotor 110 floats in the air without contacting the hydrodynamic bearing part 120 to keep smooth rotation.
For obtaining a good image quality, it is necessary to make the tilt angles of the polygonal mirror, and the accuracy of the machine-processing and the assembling process for the polygonal mirror, the rotor portion fitted with it, and the hydrodynamic bearing part should be especially high.
As mentioned above, the measures to make the tilt angles small to the utmost are dependent on making the accuracy higher for each of the parts and for assembling them, and such working process engineering or assembling process engineering requires high-precision machine tools and a high-degree technical skill, resulting in high manufacturing costs. Furthermore, if the thermal expansion coefficients of the parts are different from one another, it is inevitably difficult to keep the high accuracy stably and obtain durability.